package ru.cod.rs.x1000;

import java.io.PrintWriter;
import java.util.Scanner;

public class Z1068 {
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		PrintWriter pw = new PrintWriter(System.out);
		int sum = 1;
		int a = in.nextInt();
		if (a < 0) {
			for (int i = -1; i >= a; i--) {
				sum += i;
			}
		} else {
			for (int i = 2; i <= a; i++) {
				sum += i;
			}
		}
		pw.println(sum);
		pw.flush();
		// in.close();
		// pw.close();
	}
}
